As an electric power source of a portable electronic device such as a cellular phone and a notebook-size personal computer is used a lithium ion secondary cell which has high energy density.
An electrode material of the lithium ion secondary cell consists of a positive electrode material, a separator and a negative electrode material. For the positive electrode material, an aluminum alloy foil, which has properties that electric conductivity is excellent, electric efficiency of the secondary cell is not affected, and heat generation is small, is used as a support. For example, in Patent Document 1 an aluminum alloy hard foil is proposed which contains Fe, Mn and Si, has high tensile strength and ductility, and is excellent in bending resistance.
An active material whose main component is a metal oxide containing lithium, e.g., LiCoO2, is applied to a surface of the aluminum alloy foil. An manufacturing method is as follows: each of both surfaces of an aluminum alloy foil with a thickness of about 20 μm is applied with an active material of about 100 μm thickness, and a thermal drying process is performed to remove a solvent in the active material (hereafter, simply referred to as drying step). Further, in order to increase a density of the active material, a pressing process is performed by a press machine (hereafter, the “pressing step performed by a press machine” is referred to as press processing). A positive electrode plate thus manufactured is laminated with a separator and a negative electrode plate, then wound, shaped for accommodation in a case, and accommodated in the case.
An aluminum alloy foil for a lithium ion secondary cell is generally manufactured by a semi-continuous casting method. In a semi-continuous casting method an ingot is cast from a molten aluminum alloy, an aluminum alloy plate material (foil base) with a thickness of approximately 0.2 to 0.6 mm is manufactured by hot rolling and cold rolling, and thereafter, the thickness is adjusted to approximately 6 to 30 μm by foil rolling. Note that intermediate annealing is usually performed during homogenization of the ingot or cold rolling, if needed. For example, in Patent Document 2, proposed is an aluminum alloy foil having a strength of 160 MPa or more manufactured by the semi-continuous casting method for a current collector of an electrode of a lithium ion cell.
A continuous casting method can cast/roll molten aluminum alloy continuously to yield an aluminum alloy plate (hereafter, an aluminum alloy plate manufactured by the continuous casting method is referred to as a cast plate). Therefore, according to the continuous casting method, the homogenization process and the hot rolling step which are indispensable steps in the semi-continuous casting method can be omitted, so that yield and energy efficiency can be improved allowing reduction in manufacturing cost. Representative continuous casting methods include a twin-roll type continuous casting method, a twin-belt type continuous casting method and the like. A cast plate manufactured by the continuous casting method is generally subjected to a heating process between the steps of cold rolling in order to improve rollability. For example, a method of manufacturing an aluminum alloy foil base is proposed in Patent Document 3 in which a cast plate with a thickness of 25 mm or less is obtained by the continuous casting method, then cold rolling of 30% or more is performed and thereafter a heating process at a temperature of 400° C. or higher is performed, and further an intermediate annealing at 250 to 450° C. is performed.